Change environment vars to use {{ }}

[manics]

Change environment vars to use {{ }} as reported in #53 (comment)

TASK [upgrade-distpackages : system packages | upgrade] ************************
[DEPRECATION WARNING]: Using bare variables is deprecated. Update your playbooks so that the environment value uses the full variable syntax ('{{upgrade_distpackages}}'). This
feature will be removed in a future release. Deprecation warnings can be disabled by setting deprecation_warnings=False in ansible.cfg.